Effect of process parameters and the capacity of the
microemulsion reactor
In order to achieve the maximum yield of phenolic monomers and
representative products, the process parameters such as reaction
temperature, time and the amount of catalyst were optimized by using the
microemulsion system located at point c (Figure 3). Results (Figure S4)
show that the reaction temperature and time have similar effect and
tendency on the oxidation of lignin, the maximum yield is obtained at
433 K for 4 h, achieving 119.9 mg g-1 of phenolic
monomers and generating two typical value added chemicals, i.e. ,p- hydroxy benzaldehyde and propyl-4-hydroxybenzoate with the
yield of 48.2 and 21.2 mg g-1 respectively. In
addition, the dosages of CuSO4 and
H2SO4 for lignin oxidation in this
system were also investigated (Figure S5) and the optimized dosages of
CuSO4 and H2SO4 are 0.1
mmol and 25.0 mmol L-1 respectively. It is interesting
to note that the changing trends of p -hydroxy benzaldehyde and
propyl-4-hydroxybenzoate in Figure S5b are opposite, it suggests that
the excess of H2SO4 can promote the
oxidation of p -hydroxy benzaldehyde to p -hydroxybenzoic
acid, which can be esterified easily with n -propanol under the
acidic condition.
In addition, the oxidation performance of this microemulsion system for
technical and organosolv lignins from different resources was studied
under the optimized conditions and results are summarized in Table S2.
It can be found that this microemulsion system is also appropriate to
depolymerize other organosolv lignins into aromatic compounds. 101.2 and
109.1 mg g-1 of phenolic monomers can be produced when
bamboo and miscanthus organosolv lignins are used as the substrates
respectively, while it provides 84.8 mg g-1 yield over
poplar organosolv lignin. It implies that herbaceous lignins are much
easier to be depolymerized than hard wood lignins, due to less H unit
containing in hard wood lignins,45 which accords well
with the above results about the selective tailoring of H unit. Besides,
the yields of phenolic monomers from technical lignins are lower than
those of organosolv lignins, in line with the more plentiful C-C bond
containing structure of technical lignins.5Quantitative analysis of volatile chemicals derived from bagasse
organosolv lignin oxidation under the optimized conditions is exhibited
in Figure S6 and Table S3. There are 12.7 wt. % of phenolic
products, including 52.4, 44.8 and
2.8% of H, G and S unit products. From this perspective, it can imply
that H unit in lignin can be depolymerized easily, followed by the order
of H > G > S unit in this case.